U.S. patent number 4,813,470 [Application Number 07/118,112] was granted by the patent office on 1989-03-21 for casting turbine components with integral airfoils.
This patent grant is currently assigned to Allied-Signal Inc.. Invention is credited to Feng Chiang.
United States Patent |
4,813,470 |
Chiang |
March 21, 1989 |
Casting turbine components with integral airfoils
Abstract
Method and apparatus for controlling radial solidification in
cast turbine wheel or nozzle assembly so as to produce an equiaxed
fine grain structure in a hub portion and a directionally
solidified grain structure in an integral blade portion by means of
adjustable heat shields and heating elements disposed above and
below the mold.
Inventors: |
Chiang; Feng (Cypress, CA) |
Assignee: |
Allied-Signal Inc. (Morris
Township, Morris County, NJ)
|
Family
ID: |
22376567 |
Appl.
No.: |
07/118,112 |
Filed: |
November 5, 1987 |
Current U.S.
Class: |
164/122.1;
164/125; 164/127; 164/338.1; 164/352; 164/361; 219/420;
219/425 |
Current CPC
Class: |
B22D
27/045 (20130101) |
Current International
Class: |
B22D
27/04 (20060101); B22D 027/04 () |
Field of
Search: |
;164/122.1,122.2,122,125,127,338.1,352,361 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
0104794 |
|
Apr 1984 |
|
EP |
|
582055 |
|
Dec 1977 |
|
SU |
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Primary Examiner: Godici; Nicholas P.
Assistant Examiner: Batten, Jr.; J. Reed
Attorney, Agent or Firm: Linne; R. Steven McFarland; James
W.
Claims
What is claimed is:
1. Apparatus for casting a turbine component having a central hub
with a predominately equiaxed grain structure and radially
outwardly extending blades with a predominately directionally
solidified grain structure, comprising:
a disk-shaped mold having an interior casting cavity defining said
turbine component including portions defining said central hub and
said radially outwardly extending blades,
a heat sink positioned around the outer periphery of said
disk-shaped mold and adjacent said blade defining portion of said
casting cavity,
a top thermal shield positioned adjacent the top side surface of
said disk-shaped mold, and
a bottom thermal shield positioned adjacent the bottom side surface
of said disk-shaped mold.
2. The apparatus of claim 1 wherein said thermal shields comprise a
plurality of movable elements capable of forming a variable
diameter aperture concentric with the axis of said disk-shaped
mold.
3. The apparatus of claim 2 further including a top thermal emitter
positioned above said top thermal shield and a bottom thermal
emitter positioned below said bottom thermal shield so that said
thermal shields are located between said emitters and said
mold.
4. The apparatus of claim 3 wherein said disk-shaped mold and said
heat sink lie in a first horizontal plane and said thermal shields
and said thermal emitters lie in horizontal planes spaced apart
from said first plane but parallel thereto.
5. The apparatus of claim 4 wherein said thermal emitters each
comprise a single heating element.
6. The apparatus of claim 4 wherein said thermal emitters comprise
a plurality of individually controllable electrically heated
elements positioned in a circular array concentric with the axis of
said disk-shaped mold.
7. The apparatus of claim 6 further including a process controller
means for moving the thermal shield elements in coordination with
controlling the individual heating elements so as to produce a
radial thermal gradient in said mold.
8. The apparatus of claim 1 further including means for
distributing molten metal into said casting cavity, said means
including a pouring cup, a vertical sprue, and horizontal
runners.
9. The apparatus of claim 1 wherein said mold is a thin wall
ceramic shell mold.
10. The apparatus of claim 1 further including means for agitating
the mold.
11. The apparatus of claim 1 wherein said turbine component is an
axial flow turbine wheel and said casting cavity further includes a
portion defining a grain selector located radially outwardly from
the blade defining portion of said cavity.
12. The apparatus of claim 11 wherein said grain selector defining
portion of the casting cavity is adapted to promote the formation
and growth of a single crystal into the blade defining portion of
said cavity.
13. The apparatus of claim 12 wherein said grain selector defining
portion of the casting cavity includes a helical passageway having
one end terminating adjacent said heat sink and the other end
terminating in the blade defining portion of the casting
cavity.
14. The apparatus of claim 11 wherein said grain selector defining
portion of the casting cavity contains a solid seed crystal having
one side adjacent said heat sink and another side adjacent the
blade defining portion of the casting cavity.
15. The apparatus of claim 1 wherein said turbine component is a
nozzle and said casting cavity further includes a portion defining
an outer shroud ring located radially outwardly from the blade
defining portion of said cavity.
16. The apparatus of claim 15 wherein said heat sink is a water
cooled metal chill block in contact with the outer shroud ring
defining portion of the casting cavity.
17. The apparatus of claim 15 wherein said casting cavity further
includes a portion defining a grain selector located radially
outwardly from the outer shroud defining portion of the cavity and
lying in the plane of the blade defining portion of the cavity.
18. The apparatus of claim 17 wherein said grain selector defining
portion of the casting cavity is adapted to promote the formation
and growth of a single crystal through said outer shroud defining
portion of said cavity and into the blade defining portion of said
cavity.
19. The apparatus of claim 18 wherein said grain selector defining
portion of the casting cavity includes a helical passageway having
one end terminating adjacent said heat sink and the other end
terminating in the outer shroud defining portion of the cavity
adjacent and the blade defining portion of the casting cavity.
20. The apparatus of claim 17 wherein said grain selector defining
portion of the casting cavity contains a solid seed crystal having
one side adjacent said heat sink and another side in contact with
the outer shroud defining portion of the cavity and adjacent the
blade defining portion of the casting cavity.
21. The apparatus of claim 1 wherein said turbine component is a
radial flow turbine wheel and said heat sink is a water cooled
metal chill block in contact with the blade defining portion of the
casting cavity.
22. A method of making a cast metal turbine component of the type
having a central disk with integrally formed blades extending
radially therefrom and lying generally in the plane of the disk,
comprising the steps of:
providing a disk-shaped mold having an interior casting cavity
defining said turbine component, a cooled metal heat sink adjacent
the periphery of said cavity, and heat shields adjacent the top and
bottom side surfaces of the mold,
casting molten metal into said mold,
extracting heat from said molten metal through the peripheral heat
sink while preventing substantial loss of heat from the top and
bottom side surfaces of the mold, by utilizing said heat shields
thereby forming a radial thermal gradient in said molten metal,
and
causing said molten metal to directionally solidify radially
inwardly from said heat sink to form a columnar grain structure in
at least the blade portion of the turbine component.
23. The method of claim 22 further including the steps of moving
the heat shields after the blade portion has solidified, increasing
the cooling rate of the remaining molten metal, and forming an
equiaxed grain structure in at least the central portion of the
turbine component.
24. The method of claim 23 further including the steps of providing
an array of individually controllable heating elements positioned
adjacent said heat shields and controlling said heating elements
during solidification to enhance the radial thermal gradient in the
molten metal.
25. The method of claim 24 further including the step of agitating
the mold so as to promote the formation of a fine equiaxed grain
structure in the last to solidify molten metal.
26. The method of claim 22 further including the steps of
providing single crystal seeds within said mold located in contact
with said heat sink and extending into said casting cavity,
flowing said molten metal into contact with said single crystal
seeds,
causing said molten metal to begin to solidify on the surface of
said seeds and then grow in the form of a single crystal into the
molten metal filled casting cavity by extracting heat from the
molten metal in a radial direction through the solidifying single
crystal and into said heat sink.
27. The method of claim 26 further including the steps of
waiting until the single crystal seeds have grown the portion of
the casting cavity which defines the blades of the turbine
component and then interrupting the further growth of the single
crystals while promoting the formation of equiaxed grains in the
remaining molten metal.
28. The method of claim 27 wherein the steps of interrupting and
promoting include the step of agitating the mold.
29. A method of casting a one-piece metal turbine wheel having a
cylindrical hub and a plurality of integral radially extending
blades, the metallurgical structure of said hub being characterized
by predominately equiaxed grains and that of said blades being
predominately radially aligned columnar grains, comprising the
steps of:
providing a mold having a disk-shaped casting cavity defining said
hub and said blades, and also having a heat sink adjacent the
periphery of said cavity, movable thermal shields adjacent the top
and bottom of said cavity, and thermal emitters adjacent said
thermal shields;
casting molten nickel base superalloy metal into said cavity:
extracting heat from said molten metal in a radially outwardly
direction into said peripheral heat sink while initially inhibiting
heat flow in all other directions;
solidifying the molten metal within the blade defining portion of
said cavity to form a radially aligned columnar grain
structure;
moving said thermal shields; and
cooling the molten metal within said hub defining portion of said
cavity to promote the solidification of an equiaxed grain
structure.
Description
TECHNICAL FIELD
This invention falls broadly in the field of metal founding and,
more particularly, relates to controlling solidification of a cast
turbine wheel or nozzle assembly so as to produce an equiaxed fine
grain structure in a hub portion and a directionally solidified or
single crystal grain structure in an integral blade or airfoil
portion extending therefrom.
BACKGROUND ART
One of the limiting factors in the design of high performance gas
turbine engines is the ability of the turbine wheel or nozzle
assembly to withstand the severe conditions of temperature and
stress which exist during operation of the engine.
Turbine wheels and nozzles are located immediately downstream from
the combustion area of an engine and must operate in an environment
of high temperature corrosive gases. In addition, the turbine
wheels operate under great mechanical stress due to their very high
rotational speeds--often exceeding 100,000 revolutions per
minute.
In the past, it has been generally known to cast turbine wheel
assemblies in one piece. See, for example, U.S. Pat. Nos. 3,283,377
and 3,312,449. However, even though special care was taken to
produce high quality, fine grained hub castings, defects initiated
in the airfoil portion often led to failures.
One way to reduce failures is to forge the wheel hub or disk from a
high strength alloy and then mechanically attach individual blades
to it. The individual blades can be cast with high precision and
inspected for quality before assembly thus reducing the probability
of a defect in the wheel. In addition, such small castings may be
directionally solidified in a columnar grain structure or even
solidified as a single crystal to further improve their high
temperature properties. See, for example, U.S. Pat. Nos. 3,342,455:
3,680,625; 3,714,977; 3,260,505 and 3,376,915.
However, this two-stage process is very expensive and time
consuming; so attempts have been made to achieve the improved blade
structure in an integral casting. U.S. Pat. No. 3,614,976 suggests
that rotation of a casting mold during solidification can result in
columnar grained blades and equiaxed fine grains in the hub of
turbine wheels. A different approach is suggested by U.S. Pat. No.
3,741,821 wherein a forged wheel assembly (having an all equiaxed
grain structure) is heat treated only in the blade region to allow
those grains to grow into a larger and/or columnar structure.
More recently, attempts have been made to more accurately control
the solidification of a cast wheel assembly to produce the desired
dual macrostructures. See, for example, U.S. Pat. Nos. 3,283,377;
3,312,449; 3,598,169; 4,240,495 and 4,436,485.
A major disadvantage of these prior art processes is that it is
still very difficult to precisely control the thermal gradients in
the mold, and thus the solidification process, to achieve the
desired microstructure in the as-cast turbine wheel.
It is, therefore, an object of the present invention to provide an
improved method and apparatus for controlling the solidification of
a cast disk-shaped component.
SUMMARY OF THE INVENTION
The present invention aims to overcome the disadvantages of the
prior art as well as offer certain other advantages by providing a
novel casting system which incorporates a disk-shaped mold having a
heat sink adjacent the periphery and a combination of thermal
emitters and thermal shields adjacent the top and bottom side
surfaces of the mold.
The mold is basically a conventional thin walled investment shell
mold made by dip coating a wax pattern with several layers of
ceramic. After drying, the wax is removed and the cavity prepared
to receive molten metal.
The peripheral heat sink is generally a ring-shaped water-cooled
metal chill block located adjacent the blade portion of the mold.
Its function is to ensure that heat is withdrawn from the mold in
only a radial direction thus promoting directional solidification
toward the center of the mold.
The thermal emitters are generally electric resistant heating
elements arranged preferably in several concentric circles about
the axis of the mold but in planes spaced apart from the top and
bottom sides. Each circular heating element may be individually
controlled to provide a precise amount of heat to the adjacent mold
surface. Alternately, one continuous element may be arranged in a
spiral path or a round planar heating element can be used, if
precise control of the heat input is not required.
Movable thermal shields are arranged between the heating element
and the mold so as to provide a means for accurately controlling
the amount and location of heat added or withdrawn from the mold.
The shields are preferably constructed like an iris diaphragm in a
camera shutter so that the area of the central aperture can be
adjusted to allow more or less radiant energy to pass to or be
withdrawn from the mold as desired. The shields may be water cooled
for protection from the heat and/or for use as an auxiliary heat
sink.
During operation of the casting system, molten metal is poured into
the preheated mold and allowed to solidify under conditions
carefully controlled by the combined actions of the chill block,
heating elements, and heat shields. Preferably, the actions of
these elements are controlled by a computer or other automatic
control means so that the process is consistently repeatable from
one batch of castings to the next and the desired structure easily
achieved.
The presence of columnar zones in castings has been recognized for
some time, but until recently, this type of structure was
considered a defect and not nearly so desirable as the equiaxed
structure. In recent years, however, the properties of columnar
structures have undergone re-examination and it has now been
determined that in some applications, the columnar structures are
markedly superior to equiaxed structures. For example, it has been
found that the high temperature properties of columnar structures
are superior, particularly in fracture resistance and ductility
under creep loading conditions.
Columnar structures are formed by the unidirectional growth of
dendrites during solidification. The relationship between the
dendritic structure and the columnar grains is not exact. Each
columnar grain is usually composed of more than one dendrite, and
the number may vary from a few to several hundred. The
interdendritic spacing is related to the solidification rate only.
Columnar grain size, however, may be affected by factors other than
the solidification process, such as ordinary grain growth.
Basically, the process of the present invention involves balancing
the heat flow from the molten metal to ensure that solidification
proceeds unidirectionally, at a controlled rate, from the outermost
edge of the blades inwardly towards the hub. Initially the heating
elements are on and the heat shields fully opened, supplying heat
to the entire mold to prevent any loss of heat from the top or
bottom of the mold. After solidification begins on or near the
chill block, the heat shields are slowly closed and the outer
heating elements turned off. As the solidification front is moved
inwardly, the heat shields are progressively closed and additional
heating elements deenergized. This slow, controlled radial
solidification results in directional solidification or even single
crystal grain growth in the outermost blade region of the mold.
Finally, all the heating elements are extinguished and the heat
shields fully opened to more rapidly cool the hub region and form a
fine, equiaxed grain structure. A further grain refining process,
e.g., agitation or spinning of the mold, may then be performed, if
desired.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing
out and distinctly claiming the subject matter which is regarded as
the invention, it is believed that the objects, advantages, and
features thereof may be better understood from the following
detailed description of a presently preferred embodiment when taken
in view of the accompanying drawings in which:
FIGS. 1A, 1B and 1C illustrate three types of turbine components
having integrally cast blades extending radially outwardly from a
hub;
FIGS. 2A, 2B and 2C are cross-sectional schematics showing major
elements making up the casting apparatus for producing the three
types of components shown in FIG. 1;
FIGS. 3A and 3B are vertical views illustrating the preferred
concentric layout of the heating elements; and
FIGS. 4A and 4B are vertical views of the camerashutter type heat
shield in closed and partially opened positions.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1A illustrates one type of cast turbine wheel produced by the
method and apparatus of the present invention. Airfoil shaped
blades (11), having a directionally solidified and/or single
crystal microstructure, extend from the periphery of a disk (12)
which has an equiaxed grain structure. Typically the disk (12) is
provided with a hub (14) containing an aperture (16) for fitting
around a shaft in a turbomachine (not shown). FIG. 1B shows a type
of turbine nozzle which has a hub or inner shroud ring section
(14), an airfoil blade section (11), and an outer shroud ring (15)
joining the periphery of the blades. FIG. 1C shows still another
type, a radial flow turbine wheel, which has a relatively thicker
hub (14) and blades (11) which reduce in size as they extend
outwardly from the hub. This invention is, of course, suitable for
production of other similar types of turbine components which may
vary somewhat in the details of their construction.
FIG. 2A illustrates the major elements of the casting apparatus
used to produce the turbine wheel shown in FIG. 1A. A mold assembly
(60) generally comprises a ring-shaped chill block (61) surrounding
a ceramic shell (85) which is adapted to contain and shape molten
metal. Closely adjacent the top and bottom sides of the mold
assembly (60) are movable heat shields (40,50) which may be opened
or closed to expose more or less of the ceramic shell (85).
Outboard from each of the thermal shields (40,50) is an array of
heating elements (30,70) which supply heat to the portion of the
ceramic shell (85) exposed by the shields (40,50). A process
control system (90) is preferably used to monitor and adjust the
position of the heat shields (40,50), the amount of heat supplied
by the heating elements (30,70), the amount of heat extracted by
the chill block (61) or water cooled heat shields, and other system
variables.
The ceramic shell (85) is formed by well-known methods in which a
wax or plastic pattern of the desired wheel (along with the
necessary casting sprue and runners) is dipped into a refractory
mixture such as colloidal alumina or silica, zircon or alumina sand
or other finely divided ceramic. This process is repeated
sufficiently to build up several self-supporting layers on the
pattern. After the ceramic is dry, the pattern is removed to leave
a casting cavity for receiving molten metal. The casting cavity
shown in FIG. 2A defines areas for forming the blades (21), disk
(22), and hub (24) of the turbine wheel shown in FIG. 1A. It also
has a pouring cup (82) and sprue (83) for directing molten metal
throughout the cavity. FIG. 2B illustrates a somewhat more complex
mold for forming a nozzle of the type shown in FIG. 1B. The casting
cavity has runners (84) for directing molten metal from the down
sprue (83) into areas defining a hub (24), sometimes called an
inner shroud ring, the blades or airfoils (21), and an outer shroud
ring (25). FIG. 2C illustrates a mold suitable for forming a radial
flow turbine wheel of the type shown in FIG. 1C. The casting cavity
has a central portion (24) defining a relatively large hub and edge
portions (21) defining the blades.
In each of FIGS. 2A, 2B and 2C, the ceramic shells (85) are
surrounded by a circular chill block (61), preferably made from a
metal having good thermal conductivity, such as copper, and having
internal passageways (62) for cooling water. Between the chill
block and the portions of the shell mold which define the turbine
components are additional cavities which define a grain starter
(29) or, alternately, a single crystal selector (28). The shape and
function of these relatively small cavities are well known in the
art and serve to initiate the formation of the desired crystal
structure in the first to solidify metal. Basically, one type of
single crystal selector cavity contains a helical passageway which
permits only one of the initially solidified metal grains to grow
into the main casting cavity. Alternately, a single crystal may be
formed by placing a metal "seed" (27) in the grain starter cavity
and promoting its growth into the main cavity. Directionally
solidified columnar grains may be promoted in a similar manner.
The movable heat shields (40,50) are shown more clearly in FIGS. 4A
and 4B. They are preferably formed of several individual elements
(41, 42, 43 . . . ) which move in concert with each other to
produce a variable size aperture (49) much like an iris shutter in
a camera. They preferably are made of heat conducting metal cooled
by internal running water or ceramic since they must be closely
adjacent the hot mold. On the surface of the heat shield, a thin
layer of insulating material, like graphite or carbon-carbon
composite can be used to cover the surface that is exposed to the
heating elements, so that it is protected from very high
temperature. The other side of heat shield should be exposed so
that it can absorb the heat from the Just solidified metal and
further promote the directional solidification of the unsolidified
metal. Their primary function is to control heat transfer from the
molten metal in all directions other than radially towards the
chill block. They also control the amount and location of heat
added to the mold by the overlying heating elements (30,70).
An array of heating elements (30,70) is shown more clearly in FIG.
3A. Each array is preferably compared of several individual
elements (31, 32, 33 . . . ) which can be selectively energized in
order produce a desired thermal profile in the casting mold
assembly (60). Typically, the top array (30) and bottom array (70)
are similar unless the mold configuration allows the use of a
greater or lesser number of elements. As illustrated in FIG. 2C,
the bottom array (70) may sometimes contain only a few elements
(75,76). In some cases it would be possible to utilize a unitary
spiral shaped element as shown in FIG. 3B since the thermal shields
(40,50) can regulate the exact amount of heat delivered to the
mold. The heating elements are typically electric resistance heated
bars connected to an external power source (92) and controlled by a
process control computer (90).
In operation, the ceramic shell (85) is usually preheated to a
suitable casting temperature by opening the heat shields (40,50)
and energizing the heating elements (30, 70). Preferably, the
process control computer (90) senses the temperature of various
parts of the mold by, for example, thermocouples or other well
known means.
The molten metal (81), typically a nickel base superalloy melted by
any suitable device (80), such as an induction power melting unit,
is introduced into the mold pouring cup (82), flows down the sprue
(83), and is distributed into the mold cavity. Often the entire
mold assembly is contained within a vacuum melting furnace to
prevent contamination of the molten metal.
Because of the insulating effect of the ceramic shell (85) and the
heat input to the top and bottom of the mold from the energized
heating elements (30,70), solidification of the molten metal begins
in the grain starter cavities adjacent the chill block (61). The
chill zone consists of many fine dendrites having a random
orientation. The initial freezing releases the heat of fusion,
resulting in some temperature rise locally, arresting the chill
zone formation. At the interface of the chill zone and the melt,
the dendrites begin to grow into the melt at a rate dependent upon
the amount and depth of the supercooling.
Initially, all dendrites at the chill zone-melt interface grow at
equal rates since equal supercooling is present. However, those
oriented parallel to the thermal gradient are growing into an area
of continued supercooling. Those oriented unfavorably cannot
advance as rapidly in the direction of the thermal gradient, since
only a component of the growth velocity is aligned with this
gradient. The dendrites growing parallel to the gradient, since
they have already undergone some growth, will give off a latent
heat of fusion due to the freezing process. This heat of fusion
increases the temperature at the base of the dendrites and
decreases the amount of supercooling available for growth of the
more unfavorably oriented neighbors. In this manner, the growth of
the misoriented dendrites is stifled, and only those aligned with
the thermal gradient will undergo significant growth.
When single crystal seeds (27) are used within the grain starter
cavities (29), the molten metal solidifies on the seeds and grows
into the molten metal with the same crystal structure and
orientation as the seed. Thus it is possible to preselect the
orientation of the grains in the blades by proper selection of the
seed crystals as is well known in the art.
These single crystal grains or directionally solidified columnar
grains are forced to grow inwardly from the grain starters (29)
through the blade region (11) of the casting by maintaining the
thermal gradient in the radial plane of the mold. As the
solidification front proceeds, the thermal shields.(40,50) are
slowly closed and the outermost heating elements (71,72)
deenergized. All the heat of fusion is withdrawn from the molten
metal through the solidified grains and into the chill block (61)
to be carried away by cooling water flowing through passages (62).
Also some heat from the already solidified metal may be carried
away by the overlying heat shields if it too is water cooled.
After solidification has proceeded into the disk or hub (12)
region: of the wheel, an agitation process known for producing fine
grain structure castings can be applied to form a fine equiaxed
grain structure near the center of the wheel. This may conveniently
be accomplished by completely closing the thermal shields (40,50),
deenergizing all the heating elements (30,70), and energizing a
vibrator (98) connected to the mold. After the heating elements
have cooled somewhat, the heat shields (40,50) may be opened to
allow thermal radiation to leave the mold and further increase the
cooling rate.
While the invention has been described in terms of one preferred
embodiment, it is expected that various alternatives,
modifications, or permutations thereof will be apparent to those
skilled in the art. Accordingly, it is intended that equivalents be
embraced within the spirit and scope of the invention as defined by
the appended claims.
* * * * *